The invention relates to static power frequency changers in general, and more particularly to Unrestricted Frequency Changers (UFC) and their applications, for instance to adjustable speed AC motor drives.
The Unrestricted Frequency Changer (UFC) and its adjunct static switch control for the generation of an AC wave of controlled voltage and frequency have been described in U.S. Pat. No. 3,470,447 and U.S. Pat. No. 3,493,838 of L. Gyugyi et al. These patents show how the switches in each of the static converters associated with an output phase of the load can be selectively and cyclically controlled for conduction during a predetermined time interval so as to derive and output power defined by a controlled increment of the input voltage, itself delineated between two time intervals used for shorting the output, which process results in an AC output voltage having a frequency depending upon the repetition rate of the conduction time intervals and a magnitude measured by the time period of effective conduction of each static switch. Such an unrestricted frequency changer is advantageously applied in variable speed AC drives as explained on pages 5-14, and 363-383 of "Static Power Frequency Changers" by L. Gyugyi and B. R. Pelly, published by John Wiley & Sons, 1976. In this regard, for instance, Gyugyi and Pelly have observed that the UFC has an inherent bilateral characteristic between the power source at its input and the power supply at its output, which allows a four-quadrant operation of the motor drive without costly additional circuitry.
The unrestricted frequency changer technique has become particularly attractive with the advent of modern semiconductor switches, for instance, power transistors, and GTO devices.
UFC voltage control by pulse-width-modulation (PWM) is useful for wide range variable frequency drive of the motor. One disadvantage of the PWM UFC in a motor drive, is that it causes a large amount of extrabasal components in the input current. The extrabasal components can be reduced to a certain amount by inserting a proper input filter. This approach, however, has revealed that a somewhat large size filter is required. If the filter size is increased, filter VAR rating, that is, the reactive current, insertion loss are also increased, not to mention the cost. In order to reduce the filter size, one method is to increase the carrier frequency of the PWM UFC system from a single modulation to double, triple, quadruple modulations. As the modulation frequency increases, the frequencies of the extrabasal components which have the dominant amplitudes also shift to higher range and filtering becomes easier. However, in this case, switching loss will also increase and the upper limitation will be determined by the characteristics of the switches.